Control system for variable pitch fan propulsor with reverse pitch

ABSTRACT

A coordinated fan pitch, fuel flow and fan exhaust nozzle area control for a gas turbine powered aircraft propulsor of the bypass duct, variable pitch and exhaust nozzle configuration serves to effectuate reverse pitch change through feather, by minimizing forward thrust excursion and shaft torque to obtain rapid reverse thrust response.

BACKGROUND OF THE INVENTION

This invention relates to control means for an aircraft propulsor of theclass known as Q-Fans^(TM) being developed by the Hamilton StandardDivision of United Aircraft Corporation and particularly to coordinatedmeans for controlling the pitch of the fan blades, engine fuel flow andfan exhaust nozzle area for reversing the fan blade angle throughfeather.

To more fully understand the Q-Fan^(TM), reference should be made to U.SPat. No. 3,747,343 granted to Mr. George Rosen and assigned to the sameassignee. As is the case with all controls for gas turbine power plants,it is customary to provide means for monitoring engine operations andprovide control means to convert those signals to a logic that willprovide, as best and efficient as possible, optimum engine operations.Thus the control manifests these signals to provide fast thrust responseduring take-off and landing, optimum TSFC (Thrust Specific FuelConsumption) in all cruise conditions, while preventing stall or surge,rich or lean blowout, overtemperature, overpressure and overspeedconditions.

Obviously, the incorporation of such variables as variable pitch fans,variable area exit nozzles and the like will add complexity to thecontrol system. Application Serial No. 477,532 filed on the same data byRoy W. Schneider and Kermit I. Harner, entitled "Control System forVariable Pitch Fan Propulsor" and assigned to the same assigneedescribed a reliable coordinated control that coordinates fuel flow tothe gas turbine engine and pitch change of the fan blades and the areaof the variable exit nozzle of the bypass duct so as to achieve rapidthrust modulation in takeoff and landing modes and optimum TSFC in allcruise and long duration flight conditions while providing the typicalprotection to the gas generator. In particular this aforementionedapplication discloses control means biasing the power lever schedulewith flight Mach No. to provide control of engine fuel flow, fan pitchand area of the exit nozzle in the event this variable is included. Thesurge of the fan is prevented by defining a scheduled exhaust nozzlearea which is a function of flight Mach No. and corrected engine fanspeed (N_(F) / √θ) and feeding it to a selector circuit that selects thelarger of the normal scheduled area and the minimum fan exit area whichis required to avoid surge. The fan exit area nozzle is also utilized tooptimize performance (TSFC) for long duration flight conditions. Exceptfor the condition lever which is typically employed in aircraft forstarting, shutting-off and feather, a single power lever is socoordinated to provide engine fuel flow, variable fan pitch change andvariable area exhaust nozzle control throughout the operating range.

This invention is particularly directed to means to reverse the pitch ofthe fan through feather as opposed to passing through flat or zeropitch. This presents a significant problem since the pitch of the fanjust prior to reversing is at a low positive blade angle and must moveto a higher positive blade angle to reach the feather angle position.Without anything else being done the higher positive pitch will increasethe blade loading and produce a higher positive thrust which isobviously undesirable inasmuch as this increases forward flight velocitywhere a decrease is required. Of course, once in reverse pitch maximumreverse thrust is obtained. To achieve this end we have found means tocoordinate the coordinated functions of engine fuel flow, blade angleand exhaust nozzle area so as to minimize forward thrust by judiciouslyreducing and increasing fuel flow and/or increasing exit fan nozzle areaand optimize the transient response.

Thus, in summary, without limiting the scope of this invention thesalient features are:

1. The ability to provide rapid thrust response in the takeoff andlanding conditions by coordinating fan pitch and engine fuel flow so asto optimize transient response characteristics.

2. The ability to go from forward thrust to reverse thrust (throughfeather) rapidly while maintaining satisfactory shaft torque conditionsand minimizing increase thrust excursion by coordinating fan pitch,exhaust nozzle area and engine fuel flow so as to optimize transientresponse characteristics. For example, the time calculated from digitalsimulation to obtain substantially 100% reverse thrust from 100% forwardthrust was approximately between 1.3 to 2.0 seconds. This compares witha typical engine reverse thruster that requires substantially 8 to 10seconds.

3. The ability to modulate thrust smoothly from maximum to near zerothrust in both forward and reverse range on the ground.

SUMMARY OF THE INVENTION

An object of this invention is to provide an improved control for apropulsor having a variable pitch fan powering aircraft.

A still further object of this invention is to provide control meansthat coordinates fuel flow, pitch change of the gas turbine enginedriven fan and the exhaust nozzle area of the bypass duct surroundingthe fan for reversing the pitch of the fan through feather.

A still further object of this invention is to obtain coordinatedcontrol means for reversing through feather that minimizes thrustexcursions, reduces or minimizes forward thrust in the vicinity offeather blade angle and minimizes fan drive shaft torque in thetransient regime.

Other features and advantages will be apparent from the specificationand claims and from the accompanying drawings which illustrate anembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view, partly in section and partly diagrammaticillustrating the preferred embodiment.

FIG. 2 is a diagrammatic illustration of this invention.

FIG. 3 is a diagrammatic illustration of the anticipation circuitry.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is now made to FIGS. 1 and 2 which illustrate the preferredembodiment of this invention showing the coordinating control in blankreferenced by numeral 10 having as schematically shown suitableconnecting means 12, 14, 16 connecting the actuator 18 for the variableexhaust nozzle 20 of bypass duct 22; connecting the pitch changemechanism of the variable pitch fan generally illustrated by numeral 24;and connecting the fuel control 26, respectively.

Fuel control 26 may be any well known commercially available fuelcontrol, such as the JFC-42-C or the JFC-26 model manufactured by theHamilton Standard Division of United Aircraft Corporation or the typeillustrated in U.S. Pat. No. 2,822,666 granted to S. G. Best on Feb. 11,1958 and also assigned to the same assignee suitably modified tocomplement this invention. Suffice it to say that the fuel controlserves to meter fuel to the engine as a function of the control logicand serves to prevent surge, overtemperature and overpressurization in awell known manner. However, the control, according to the teachings ofthis invention would, of necessity, be suitably modified to reflect theutilization of the parameters monitored by and converted into controllogic by the control 10. This aspect of the invention will be more fullyappreciated from the description to follow, but one skilled in this artwould have no difficulty in applying the teachings of this invention tothe established fuel control technology.

In this instance, the fuel control 26, receiving the control logic fromthe coordinated control 10 serves to meter the proper amount ofpressurized fuel received from pump 28 to the burner section of the gasturbine engine via line 30. Preferably fuel control 26 will provideprotection for the gas generator so as to prevent overtemperature,overtorque, overspeed and overpressure conditions, as well as avoidingsurge and flame out as power is varied.

While not specifically limited thereto, as will be appreciated by oneskilled in the art, the preferred embodiment contemplates a gas turbineengine having a free turbine, i.e., the free turbine is not mechanicallyconnected to the gas generator and its sole connection is through theaerodynamic coupling of the gases which flow through the compressor andturbine sections.

The pitch change mechanism shown in blank by reference numeral 34responding to the coordinated control 10 via connection 14, may be anysuitable type and for the sake of convenience and clarity, a detaileddescription thereof is omitted. Suffice it to say that the pitch changemechanism serves to vary the blade angle (β) of fan blades 36 which aresuitably rotatably supported in hub 38 in any well known manner. In thisembodiment because of the high blade solidity factor and the responsecharacteristics, it is contemplated that the blades are reversed throughfeather, although such a requirement is not germane to this invention.If, however, that is the case, it would be desirable to provide a highpitch stop, short of feather, to prevent an inadvertent feather. Such astop would be, in concept, similar to the low pitch stop customarilyprovided in all propellers that are powered by gas turbine engines, asfor example, the 54H60 propeller manufactured by the Hamilton StandardDivision of United Aircraft Corporation.

Referring to FIG. 1 it will be appreciated that the control 10 receivessignals from a pair of control levers 40 and 42, one being the conditionlever which is utilized for starting, feathering and shutting-off andthe power lever, respectively. The conditioning lever and its functionsare well known and since it is not deemed a part of this invention, adetailed description is omitted for the sake of convenience andsimplicity.

The power lever (PLA) serves to provide the input to the control so thatthe control 10 will automatically set the power of the gas generator toprovide the necessary aircraft operating conditions in both forwardflight and reverse modes. The control 26 is designed to provide rapidthrust modulation in the takeoff and landing modes and optimum TSFC inall cruise and long duration flight conditions.

Reference will next be made to FIG. 2 which schematically describes thecontrol logic contemplated for effectuating the above. Ignoring for themoment the reversing interlocks 44, 46 and 48, it will be noted that thepower lever schedules free turbine speed (N_(F) ref) by generating abiased signal responding to flight Mach No. (M_(N)) and compressor inlettemperature (T₂) in the function generator 50. The actual free turbinespeed (N_(F)) is compared with N_(F) ref in summer 52 which signal ispassed to the governing compensation network 54. Network 54 is asuitable well known proportional plus integral fan speed governorserving to modulate the fuel within the conventional fuel constraintsscheduled by control 26.

It is apparent from the foregoing that N_(F) is set as a function ofPLA, T₂ and M_(N) and speed governing through the proportional plusintegral fan speed governor modulates fuel flow to maintain the speederror at zero.

In order to compensate for the inertia of the gas generator turbine andcompressor and obtain fast thrust response by pitch change it isdesirable to incorporate an anticipatory circuit. This anticipatorysignal responding to PLA rapidly changes fuel flow relative to the slowchanges in N_(F) so as to provide the change in power needed to hold thedesired speed of the fan while fan pitch is changed. This anticipatorysignal which may be a suitable derivative signal, i.e. a reset which isproportional to the velocity of the input signal, minimizes fan speedexcursions and helps to improve the thrust response characteristicsduring transient conditions.

While a well known K S/τS + l type of mechanism may be employed as theanticipatory signal, FIG. 3 describes a preferred embodiment foreffectuating the anticipatory function. In this instance the basicproportional and integral speed control is obtained by time integrationof a derivative and proportional signal of speed error. Similarly powerlever anticipation signal is obtained by time integration of aderivative signal.

In either instance, anticipation is a function of PLA and flight M_(N),so that below a predetermined PLA and above a predetermined M_(N) noanticipation will occur. This is represented by the curve in box 56 ofFIG. 2. Thus at the point of the curve in function generator 56 wherethe curve becomes horizontal, the scheduled output is constant and henceno anticipation signal will ensue. It is only when the PLA reaches apredetermined value, that anticipation will occur and will generate asignal which will pass to the anticipation compensation circuit 58. Thecurve in box 56 is shaped to provide anticipation only in the low M_(N)forward thrust regime where rapid thrust modulation is required fortake-off and landing.

Hence, since power lever can change very rapidly, the speed error andanticipation derivative can become very large magnitude - short timeduration signals. To avoid loss of a portion of these signals due tounavoidable signal limiting these derivative signals are converted intosmaller amplitude - longer duration signals. This is accomplished byproviding a rate - limited first order lag and then computing thederivative of this lag output as shown in FIG. 3. The combination ofsummer 60, high gain 61, limits 62, and time integration 63, form a ratelimited first order lag such that the output from 63 follows theanticipation signal from box 64 with a defined maximum rate. It isobvious that the derivative of the time integration may be obtained fromthe input to the integration, thus the signal to the integration intobox 63 is also passed through the anticipation gain 65 to yield the PLAanticipation signal.

The fan speed governor is basically a conventional proportional andintegral control. This is mechanized in FIG. 3 by forming the speederror 66 from the difference between N_(F) REF and N_(F) SENSED, thenpassing this error signal through a proportional gain 67 and integralgain 68. A rate-limited derivative of the proportional signal is formedin essentially the same manner as used in the anticipation circuit,except that the integration 72 has magnitude limits to permit smoothtransition from fuel limiting to speed governing. Specifically, thecombination of 69, 70, 71, 72 form a rate-limited lag that is alsomagnitude limited in box 72. The derivative 73 of the box 72 output isadded to the speed integral signal from box 68 and the anticipationsignal from box 65 in summation 74. The output from summation 74 is ameasure of the desired rate of change of engine fuel. This signal ispassed to the fuel control and time integrated 76 (within the fuelconstraints 75 scheduled in the fuel control) for effecting changes inmetered fuel flow.

Referring back to FIG. 2, PLA and flight M_(N) are used in functiongenerator 70 to generate a commanded steady state fan blade anglesignal, B_(RSS). The fan blade angle is adjusted in a suitable manner toachieve this B_(RSS).

Likewise, the PLA and flight M_(N) are used in function generator 72 tocreate a commanded optimum fan exhaust nozzle area signal, A_(FSS). Itis apparent from the foregoing that PLA and flight M_(N) are used toschedule fan blade angle and fan exhaust nozzle area in order tooptimize performance in the normal operating regime.

In order to assure that the fan operates without excursions into thesurge range the function generator 74 is provided. This schedules A_(F)as a function of corrected N_(F) and M_(N) (NF/√θ where ##EQU1## Thisgenerated signal i.e., the output of function generator 74 is comparedwith the scheduled A_(F) signal generated by the function generator 72by maximum selector switch 76, permitting solely the higher of the twovalues to pass through, A_(FFWD). The area of exhaust nozzles 20 (A_(F))are adjusted in a suitable manner to achieve this area, A_(FFWD).

Thus, by virtue of this invention fan speed, fan pitch and fan nozzle iscoordinated in such a manner as to optimize TSFC in the normal flightregimes and to optimize thrust response at takeoff and landing modes.The control minimizes complexities by being compatible with existingtypes of controls that already have provision for preventing surge,overtemperature, include acceleration and deceleration schedules andhave overpressure and overspeed limits necessary for gas turbine engineoperation.

As was emphasized above, by virtue of this requirement of reversingthrough feather, the fan in order to accomplish this feature wouldincrease thrust until feather position is reached as well as increasingthe torque on the fan driving shaft. This, obvious, is counter to whatis necessary for good braking characteristics that are desired foroptimum short aircraft landing performance.

Thus, in order to obtain rapid reverse thrust without exceeding shafttorque limits and minimize any increased forward thrust excursions, thefan pitch, fuel flow and fan exhaust nozzle area are coordinated duringthe transition to reverse thrust. To this end interlocks 44, 46 and 48are included in the speed, blade angle and nozzle area circuitry. Theinterlock 44 includes a fan pitch (β) override, interlock 46 includesfan exhaust nozzle area (A_(F)), fan speed (N_(F)), fan pitch (β), andpower lever angle (PLA) overrides and interlock 48 includes fan pitch,fan speed and power lever angle overrides so as to perform the followingcontrol logic.

When reverse thrust is requested by retarding the power lever angle tothe reverse thrust range, the fan pitch is scheduled to go to itsreverse pitch and fan nozzle area is scheduled to the reverse position.In order to minimize the increased thrust and shaft torque transientwhich results from increasing fan pitch, fan exhaust nozzle area isopened as rapidly as possible and fan speed reference is decreased whichcauses engine fuel flow to decrease to the deceleration limit. Duringthis initial time interval, fan pitch is either held fixed for a shorttime or allowed to increase at some relatively slow rate until theexhaust nozzle area has opened to a prescribed value and fan speed hasdecreased sufficiently to minimize the increased thrust transient. Afterthe fan pitch has increased past the feather angle, the blades areallowed to go to their scheduled reverse angle at maximum rate and theN_(F) reference is restored to its normal scheduled reverse thrustspeed.

The unreversing coordination of engine fuel flow, fan pitch and exhaustnozzle area need be slightly different to prevent fan overspeed andexcessive shaft torque in the feather region. When unreversing thrust isrequested by advancing the PLA to the forward thrust regime, the fanpitch is scheduled to go to its forward pitch position, and the fannozzle area is scheduled to its area position associated with forwardthrust. The fan pitch is decreased to forward pitch as rapidly aspossible, and the nozzle area is delayed at reverse area position toavoid fan surge in the feather region. The PLA signal, as used infunction generator 50 and 56 is temporarily reset to a low power toavoid fan overspeed and excessive shaft torque while the fan pitch ismoving from reverse to forward pitch. The PLA schedule returns tonormal, and the exhaust nozzle area is allowed to move to the forwardarea position after the fan pitch has passed through the feather region.

While not specifically limited thereto, the preferred embodimentcontemplates the execution of the concept disclosed herein by use ofwell known digital type of electronic controllers.

It should be understood that the invention is not limited to theparticular embodiments shown and described herein, but that variouschanges and modifications may be made without departing from the spiritor scope of this novel concept as defined by the following claims.

We claim:
 1. For a ducted fan propulsor for aircraft having variablepitch blades movable to a reverse position through feather and driven bya free turbine of a turbine type power plant, coordinated control meansfor controlling the pitch of said blades and the flow of fuel to saidpower plant, said coordinated control means for controlling fuel flowincludes scheduling means responsive to power lever position, flightMach No. and power plant inlet temperature for producing a first signal,means responsive to free turbine speed for producing a second signal,means responsive to said first signal and said second signal forproducing an output signal for controlling fuel flow, said coordinatedcontrol means for controlling pitch of the blades being responsive topower lever position and flight Mach No., said coordinated control meanshaving reverse power interlock to reduce forward thrust during a landingoperation, to coordinate the fuel flow and reverse pitch by having saidfuel flow further controlled by the blade angle of said blades and thepitch controlled by power lever position, and an engine operatingparameter.
 2. For a ducted fan propulsor as claimed in claim 1 whereinsaid engine operating parameter is power plant operating speed.
 3. For aducted fan propulsor as claimed in claim 1 wherein said pitch is alsoresponsive to actual blade angle.
 4. For a ducted fan propulsor foraircraft having variable pitch blades movable to a reverse positionthrough feather driven by a free turbine of a turbine type power plantand variable area duct exit nozzle coordinated control means forcontrolling the pitch of said blades and the flow of fuel to said powerplant and the area of said nozzle, said coordinated control means forcontrolling fuel flow includes scheduling means responsive to powerlever position flight Mach No. and power plant inlet temperature forproducing a first signal, means responsive to free turbine speed forproducing a second signal, means responsive to said first signal andsaid second signal for producing an output signal for controlling fuelflow, said coordinated control means for controlling pitch of the bladesbeing responsive to power lever position and flight Mach No., saidcoordinated control means being responsive to flight Mach No. and powerlever position for producing a scheduled signal for controlling the areaof said nozzle, said coordinated control means having reverse powerinterlock to reduce forward thrust during a landing operation prior toattaining feather and passing to reverse by coordinating fuel flow,reverse pitch and nozzle area by having said fuel flow furthercontrolled by the blade angle of said blades, the pitch furthercontrolled by power lever position blade angle and engine power plantrotational speed and the area of the nozzles further controlled by bladeangle, power plant rotational speed and power lever position.